The present invention relates to an electronic automatic speed control system for automotive vehicles, and more particularly to an improvement in an electronic speed control circuit of the type in which a memory in the form of a capacitor is provided to couple a speed signal proportional to an actual vehicle speed to a high input impedance terminal of a high input impedance amplifier to memorize a command signal therein.
In conventional electronic speed control circuits of this kind, as disclosed in the U.S. Pat. No. 3,455,411 patented on July 15, 1969 and U.S. Pat. No. 3,895,684 patented on July 22, 1975, a memory capacitor is connected at one end thereof to a speed signal generator and at the other end thereof to the conrol electrode of a high input impedance amplifier in the form of a field effect transistor to memorize a command speed signal. In the operation of the speed control circuits, a command speed signal is stored in the memory capacitor by momentarily connecting the common terminal between the capacitor and the high input impedance amplifier to a given voltage so that the command speed signal is memorized in the capacitor over a long period of time by means of the high input impedance property of the field effect transistor. Thereafter, an error signal developed in the field effect transistor is amplified and applied to a vacuum modulator which controls the pressure in a vacuum motor which, through a linkage, positions the vehicle throttle.
In the above-mentioned memory circuit, it is, however, observed that over a period of time the electric potential of the capacitor gradually increase at the connecting side of the field effect transistor, because the electric current fed to the drain of the field effect transistor, leaks slightly toward the capacitor from the gate of the transistor. As a result, the memorized command speed signal voltage becomes large in value so that the actual controlled speed of the vehicle becomes lower than the command speed. Furthermore, in the above-mentioned speed control circuit, a negative feedback circuit is provided to be regulated by a potentiometer which is actuated in response to movement of the vacuum motor such that the feedback circuit acts to regulate the error signal generated from the field effect transistor so as to improve response and stability of the speed control circuit. It is, however, also observed that the actual speed of the vehicle maintained by the speed control circuit is gradually decreased due to regulation of the error signal unless compensation means is adapted to the speed control circuit to compensate for the function of the feedback circuit.